The immunologic specificity of the protozoan Paramecium aurelia has been shown to be the result of a surface associated protein (termed immobilization antigen) whose antigenic specificity is under the control of cytoplasmic and nuclear determinants. While researchers agree that the protein responsible for the immobilization reaction is a large, fibrous protein of 320,000 molecular weight, there is conflicting evidence concerning the existence and number of subunits. The existence or lack of subunit structure will give insight on the gene(s) responsible for their production and aid in understanding cross- reactions observed in gel diffusion tests. Methods of purification are currently being developed which allow for the extraction and purification of myoglobin from Paramecium also (Z01 AM 25,002,02 LCB).